Magnetic storage media are widely used in various applications, particularly in the computer industry for data/information storage and retrieval applications, as well as for storage of audio and video signals. Disc drive memory systems store digital information that is recorded on concentric tracks on a magnetic disc medium. At least one disc is rotatably mounted on a spindle, and the information, which can be stored in the form of magnetic transitions within the discs, is accessed using read/write heads or transducers. A drive controller is typically used for controlling the disc drive system based on commands received from a host system. The drive controller controls the disc drive to store and retrieve information from the magnetic discs. Efforts are continually made with the aim of increasing the areal recording density, i.e., bit density of the magnetic media.
Referring to the drawings wherein identical reference numerals denote the same elements throughout the various views, FIG. 1 illustrates a perspective view of a typical disc drive data storage system in which the present invention is useful, and also a disc recording medium and a cross section of a disc showing the difference between longitudinal and perpendicular magnetic recording. Although FIG. 1 shows one side of the disc, magnetic recording layers are usually sputter deposited on both sides of the non-magnetic aluminum substrate of FIG. 1. Also, although FIG. 1 shows an aluminum substrate, alternative embodiments include a substrate made of glass, glass-ceramic, aluminum/NiP, metal alloys, plastic/polymer material, ceramic, glass-polymer, composite materials or other non-magnetic materials.
Magnetic thin-film media, wherein a fine grained polycrystalline magnetic alloy layer serves as the active recording medium layer, are generally classified as “longitudinal” or “perpendicular,” depending on the orientation of the magnetization of the magnetic domains of the grains of the magnetic material. In longitudinal media (also often referred as “conventional” media), the magnetization in the bits is flipped between lying parallel and anti-parallel to the direction in which the head is moving relative to the disc. Perpendicular magnetic recording media are being developed for higher density recording (without experiencing the thermal stability limit) as compared to longitudinal media. The thin-film perpendicular magnetic recording medium comprises a substrate and a magnetic layer having perpendicular magnetic anisotropy (Hk). In perpendicular media, the magnetization of the disc, instead of lying in the disc's plane as it does in longitudinal recording, stands on end perpendicular to the plane of the disc. The bits are then represented as regions of upward or downward directed magnetization (corresponding to the 1's and 0's of the digital data).
While perpendicular media technology provides higher areal density capability over longitudinal media, granular perpendicular magnetic recording media is being developed for further extending the areal density as compared to conventional perpendicular magnetic recording, which is limited by the existence of strong lateral exchange coupling between magnetic grains. Granular structures provide better grain isolation through oxide segregation to grain boundary, hence enhancing grain to grain magnetic decoupling and increasing media signal to noise ratio (SNR). A granular perpendicular magnetic layer contains magnetic columnar grains separated by grain boundaries comprising a dielectric material such as oxides, nitrides or carbides to decouple the magnetic grains. The grain boundaries, having a thickness of about 2 angstroms to about 30 angstroms, provide a substantial reduction in the magnetic interaction between the magnetic grains.
Perpendicular media have replaced longitudinal media in many computer-related recording applications and continue the movement toward ever-increasing areal recording densities far beyond the capability of longitudinal media. However, perpendicular media and recording technology are less well developed than facets of longitudinal media and recording technology.
As recording density continues to increase, it is necessary to make smaller grain structures to maintain the number of magnetic particles in a bit at a similar value. However, smaller grain structures are easier to erase, requiring higher anisotropy to maintain thermal stability, and making writability worse. One of the dominant issues facing perpendicular recording is unintentional erasure, particularly sidetrack erasure (STE). It is therefore increasingly difficult to make recording media that can be effectively written on the recording track by the recording head, while avoiding erasure of neighboring tracks by stray fields or limited thermal stability.